All posts tagged: relay

Wireless communication from a sender to a receiver is significantly affected by the interference generated by other devices. If devices in the vicinity of the receiver transmit at the same time and on the same frequency channel, their signals interfere at the receiver with the intended signal from the sender and thus impede proper reception.

The key idea behind cooperation in wireless networks is that devices help each other to communicate messages properly over the air. When a transmitter sends messages to a receiver, adjacent devices can overhear these messages. If the direct transmission to the receiver fails, one of those devices can retransmit (“relay”) its message copy to the receiver. It was shown that such relaying can outperform standard communication techniques. In practice, however, coordination protocols among the involved devices are needed, in particular solutions for relay selection and medium access control. Helmut Adam, Evsen Yanmaz, Christian Bettstetter, and Wilfried Elmenreich have been working on this challenge for four years. They specified and evaluated the COREMAC protocol targeted for low-budget and energy-constrained off-the-shelf hardware. The protocol located at the Medium Access Control (MAC) layer integrates radio resource reservation, relay selection, and packet flow. “We believe that cooperative relaying will not be implemented in practice until protocols supporting the physical layer to exploit the benefits of relaying are designed and properly specified,” researcher Adam says. Bettstetter adds: “The challenge of …

Wireless communications often suffers from error bursts — a long sequence of bits is corrupted when being transmitted over the air. A relatively new technique that tries to mitigate this unreliability is cooperative relaying. Its basic idea is simple: When a device transmits data packets to a destination, adjacent devices can overhear these packets. If the direct transmission to the destination fails, one of those devices can retransmit (“relay”) its packet copy to the destination. Information theorists have proved that such relaying can outperform standard communication techniques. In practice, however, relaying requires some coordination among the involved devices. In particular, a protocol for relay selection is used which in turn causes extra signaling packets. Such overhead can reduce the throughput benefits gained by relaying, and a relay update policy must define how and when relay selection is performed anew. Nikolaj Marchenko and Christian Bettstetter investigate the performance of cooperative relaying with relay selection. Their forthcoming article to be published in the IEEE Transactions on Vehicular Technology proposes a framework for modelling and analysis of cooperative …

Researchers at Klagenfurt’s Lakeside Labs work on self-organizing wireless networks which should eventually allow communication between vehicles. Günther Brandner was interviewed by Markus Böhm from the Austrian daily newspaper Der Standard. “Cars are already computers on wheels. We just need to make them talk to teach other,” he says. The team is currently testing the performance of protocols in real-world road traffic which provides new insight on how to optimize them. Read online or download:

Cooperative relaying has been developed for wireless communications to mitigate the negative effects of small-scale fading caused by multipath propagation. A huge amount of research has been done in the past ten years to assess benefits and drawbacks of such techniques by simulations and analytical means. It is surprising, however, that only few studies with real-world measurements in realistic environments were published so far. The goal of a research team led by Christian Bettstetter at Klagenfurt’s NES institute is to contribute toward closing this research gap. Based on an implementation of a simple cooperative relaying protocol on the programmable radio platform WARP, measurements were conducted to evaluate the packet delivery performance in a car-to-car communications scenario. The results will be published in IEEE Wireless Communications Letters. “We studied the ratio and temporal correlation of packet delivery for suburban and highway environments using three cars serving as sender, relay, and destination,” Günther Brandner, a researcher in the project team, explains.

Some protocols and applications in mobile computing require that a device knows as to how many other devices exhibiting a certain attribute are in its radio coverage. Obtaining this cardinality information by message exchange between devices is reliable but inefficient in dense networks in terms of overhead and delay. Performing an estimation of the cardinality using probabilistic trials is an alternative. Adam, Yanmaz, and Bettstetter pursue such a probabilistic approach by proposing cardinality estimator protocols that require no coordination among polled devices but are based on a simple random access scheme with busy tones exploiting the number of empty slots to infer about cardinality. Their forthcoming article in IEEE Transactions on Mobile Computing compares three estimators with different levels of adaptability and feedback from the query device and discusses suitability for IEEE 802.11 and low power sensors. Download article:

Helmut Adam will receive the dissertation award of the Austrian Society for Information and Communication Technologies (GIT-OVE) for his dissertation on cooperative diversity protocols in wireless networks. The award ceremony will take place in Vienna in May 2012. Adam was research and teaching staff member in Klagenfurt’s mobile systems group from mid 2006 until mid 2011 and is now a senior engineer at easyplex software.

Freezing rain causes an accident on a highway. Sensors attached to cars detect this event and communicate it via WLAN to other cars to inform oncoming drivers. The implementation of new WLAN technologies for car-to-car communication is running at full speed. Bettstetter and his team have also been tackling this problem. Their key ideas lead to filing two patent applications. Esther Farys reports. Download article:

How to improve the robustness of WLAN communications? Researchers at Klagenfurt’s mobile systems group lead by Christian Bettstetter aim at answering this question. “I expect performance gains using the concept of cooperation between devices,” the 38-year old professor says. If data transmission between a notebook and a WLAN station fails, another notebook or any mobile device will be of assistance. “Sounds simple but in fact is quite an engineering challenge,” senior researcher Wilfried Elmenreich points out. It took the project team several months to develop a communication protocol that realizes the concept of cooperation. “The backwards compatibility to existing IEEE 802.11 WLAN systems is an important feature,” PhD candidate Helmut Adam adds. The ideas of the three scientists were filed as patent applications at the European and US patent offices. The manuscripts have recently been published and are available for download using identifiers EP 2326029 and EP 232630. The research team is now busy implementing the communication protocol on a programmable hardware platform to field-test and evaluate the inventions. Students who would like to join …